The invention relates generally to systems and methods for the inspection of manufacturing processes and, more particularly, is directed to an integrated inspection system and defect correction process for micro-replicated display film manufacture.
In backlight computer displays or other display systems, optical films are often used to direct light. For example, in backlight displays, light management films use prismatic structures (often referred to as microstructure) to direct light along a viewing axis (i.e., an axis substantially normal to the display). Directing the light enhances the brightness of the display viewed by a user and allows the system to consume less power in creating a desired level of on-axis illumination. Films for turning or directing light can also be used in a wide range of other optical designs, such as projection displays, traffic signals, and illuminated signs. The prismatic structures are generally formed in a display film by replicating a metal tool, mold, or electroform having prismatic structures disposed thereon, via processes such as stamping, molding, embossing, or UV-curing. It is generally desirable for the display film and the mold to be free from defects so as to facilitate a uniform luminance of light. Because such structures serve to strongly enhance the brightness of a display, any defects, even if they are small (on the order of 10 microns), can result in either a very bright or very dark spot on the display, which is undesirable. The mold and the display films are therefore inspected to eliminate defects.
Molds, for example, electroforms are generally used for manufacturing light management films, such as prism sheets, for use in liquid crystalline displays. In general, such light management films have at least one microstructured surface that refracts light in a specific way to enhance the light output of the display. Since these films serve an optical function, the surface features must be of high quality with no roughness or other defects. This microstructure is first generated on a master, which may be a silicon wafer, glass plate, metal drum, or such; and is created by one of a variety of processes such as photolithography, etching, ruling, diamond turning, or others. Since this master tends to be expensive to produce and fragile in nature, tooling or molds are typically reproduced off of this master, which in turn serve as the molds from which plastic microstructured films are mass-produced. These tools can be metal copies grown via electroforming processes, or plastic copies formed via molding-type processes. Tools copied directly from the master are called 1st-generation, copies of these tools are called 2nd-generation, etc. In general, multiple copies can be made of every tool made at any generation, leading to a geometric growth in number of tools with each generation—i.e. a “tooling tree” is produced. Each generation is an inverted image of the previous generation. If the desired final product is a “positive” geometry, then any generation of tooling that is a negative can be used as a mass-production replication tool. If the master is manufactured as a negative, then any even-generation mold can be used for mass-production.
One difficulty always present when a manufacturing process, such as the optical display film manufacturing process, uses a component or subprocess in a subsequent step of the process is the systemic defect. If a major component, such as a shim tool or a master tool, is defective, then every subsequent mold and film replicated from those components will be defective. In prior attempts to alleviate this problem, the optical display film manufacturing process has been separated into three semi-independent manufacturing processes, the master tool, the shim tool and the display film manufacturing processes. Each primary manufacturing process has had an independent inspection and defect correction process that identifies a defective component or product at that particular step in the process and then removes it from the process chain. These processes are intended to prevent a defective master tool from being made into a defective shim tool, a defective shim tool from being made into defective film samples, and defective film samples from being sold.
However, not all the defects that will eventually make defective film are found in the inspection and defect correction processes of the master tool and the shim tool manufacture. Also the master and the shim tools eventually wear out and have to be replaced and some defective or questionably defective display film may enter the production chain before the defect is identified and corrective action taken. Most notably, in the step of the process at which systemic defects are best identified, the film inspection process, it is very difficult to collect sufficient and relevant data with human inspectors and prohibitive to fully inspect every product unit.
Therefore, there is a need to be able to better identify systemic defects in the manufacturing process for optical display films and to advantageously use such systemic defect information to correct the process in earlier steps so as identify the root cause of those defects. Such integration of the systemic defect information with corrective actions in one or all of the primary process steps will assist in eliminating or reducing the possibility of such defects in the future.
Further, the prior inspection systems and defect correction processes of the manufacturing process for microreplicated films have some disadvantages that need to be overcome. Industry practice presently involves intensive human visual inspection in order to control the quality of microreplicated optical display films. Human inspectors are very good at determining the classification of a defect, even across a complex decision matrix of defects that are objectionable within an LCD display. They, however, are not very fast at scanning and identifying many possible defects across large numbers of products for later classification and decision.
Moreover, there tends to be at the possible defect identification stage more of a subjective determination with many human inspectors collecting a large amount of data. One of the more important data for defect correction in the microreplicated optical film process is the location of a defect, as many defects across the products in a similar or the same location would tend to show a systemic defect such as a defective master or shim tool. Reliance on human inspectors makes data collection of defect position information difficult because such details require extra time in an already costly inspection process.
Because the primary useful characteristics, and thus defects, of the display film are optical, the inspection processes of these products tend to lend themselves to automated or machine vision systems. However, the use of machine vision systems have not generally been accepted in the LCD backlight industry as a superior method for quality control due to its limited flexibility in detecting the wide range of optical defects that are objectionable in an LCD display film. Machine vision systems are very efficient at making initial inspections and identifying many possible defects over many products by rapidly scanning and storing data about such defects including their location. These automated inspections can also be made by a machine vision system that has the advantage of operating without stopping production.
Therefore, there is a need to coordinate and integrate the machine vision systems and human inspections in the manufacturing of optical display film in order to use their individual strengths to advantage. This would produce a synergistic effect and provide an improved quality control process for the optical display manufacturing process whereby large numbers of product can be easily and quickly scanned to identify defects and then have actual defects classified visually in an accurate manner.
With recent improvements to machine vision systems, including multiple resolution systems where rapid scans at low resolutions can be made to locate possible areas of interest and then slower scans of the identified areas at higher resolution can be made to classify such areas, these tools should become even more practical in optical display film inspection process. There is, however, substantial difficulty in setting the machine vision system operating parameters in any given manufacturing process, particularly for an optimum scan speed and resolution. Each manufacturing process has its own particular set of defects that it is trying to capture and there is always a trade off between manufacturing efficiency and quality.
Therefore, there is a need to provide a method for more accurately controlling the operating parameters and classification rules of a machine vision system in the display film manufacturing process to reduce the number of false positive defects for efficiency sake while accurately classifying most or all of actual defects for quality sake.